In recent years, from the viewpoint of environmental protection, attention has been paid to poly(lactic acid) resins, and especially to polylactic acid resins as plant-based carbon neutral materials. Polylactic acid resins have melting points of as high as about 170° C. and can be processed by melt-molding. Further, since lactic acid, which is the monomer for those resins, can now be produced inexpensively by a fermentation method using microorganisms, polylactic acid resins are expected as bioplastics which can replace the petroleum-based commodity plastics, and gradually becoming common.
Major methods of producing polylactic acid resins are the ring-opening polymerizetion method by polymerization of lactide, which is a lactic acid dimer, by ring-opening, and the direct polycondensation method by dehydration polycondensation using lactic acid. The direct polycondensation method is said to be capable of more inexpensively producing a polylactic acid resin compared to the ring-opening polymerization method since the step of synthesizing lactide is not necessary and lactic acid can be directly used as a polymerization raw material.
JP 8-183840 A, JP 2000-297145 A, JP 2000-297143 A, JP 11-106499 A, JP 2000-302852 A, JP 2001-192444 A, JP 2001-064375 A, JP 2009-144132 A, JP 2000-273165 A and WO 2009/142196 A1 describe the direct polycondensation method. JP '840, JP '145 and JP '143 describe methods of producing polylactic acid by direct melt polymerization. JP '499 discloses a production method by combination of melt polymerization and solid-phase polymerizetion. JP '852, JP '444, JP '375 and JP '132 also disclose production methods by combination of melt polymerization and solid-phase polymerization. JP '165 and WO '196 also discloses production methods by combination of melt polymerization and solid-phase polymerization.
In the techniques described in JP '840, JP '145 and JP '143, there is a problem in that the obtained molecular weight is low and use of a solvent is necessary for obtaining a high-molecular-weight product. Further, in the method of JP '143, there is the problem of coloring of the polymer.
In the technique described in JP '499, the molecular weight after solid-phase polymerization is insufficient even when the molecular weight before the solid-phase polymerization was high.
In the techniques described in JP '852, JP '444, JP '375 and JP '132, since crystallization in water, crystallization for a long time, or crystallization with hot air containing moisture is carried out before solid-phase polymerization, acidic compounds increase in the polymer so that the rate of solid-phase polymerization and the yield of the polymer decrease, which is problematic.
In the technique described in JP '165, crystallization before solid-phase polymerizetion is insufficient and a sufficient rate of solid-phase polymerization cannot be obtained. In the technique described in WO '196, contacting with air occurs for a long time during the process from melt polymerization to solid-phase polymerization because of pulverization and the like, leading to an increase in acidic substances and, hence, resulting in a decreased solid-phase polymerization efficiency.
It could therefore be helpful to provide a method of efficiently producing a poly(lactic acid) resin having a high molecular weight and, preferably, having a high melting point as well as excellent thermal stability and hue.